The present invention relates to an electromagnetic or magnetic actuator, particularly for actuating circuit breakers in electric power distribution networks.
More specifically, the present invention relates to an actuator of the bistable type, i.e., having two fixed positions, each of which can be maintained permanently, and comprising at least one permanent magnet. Preferably, the actuator according to the invention is used to actuate a gas-insulated (e.g. SF.sub.6) circuit breaker.
Bistable electromagnetic actuators, by means of the presence of permanent magnets in their structure, are capable of applying considerable holding force in the two positions, and this force must be overcome in order to move the actuator element.
It is therefore necessary to generate an intense magnetic field, usually by means of windings, in order to overcome the field of the permanent magnets and produce a sufficiently intense separation force.
Documents of the prior art disclose a bistable actuator of the permanent-magnet type, which comprises a magnetic yoke having a laminar structure, at least one permanent magnet, an armature which can move axially between a first stable position and a second stable position, and means for moving the armature between said first and said second stable positions. The locus of the force/position characteristics is plotted in FIG. 3. Prior art actuators are generally focused on the solution of problems deriving from the need of having a high holding force, without considering problems associated with friction forces.
One problem of magnetic actuators arises from the mechanical force/position characteristic generated by the mechanism to be actuated, and in particular from the friction forces which are associated with the breaker being actuated and which must be overcome for the correct operation of said devices.
The friction forces are static and/or dynamic forces and may be present only in particular sections of the stroke of the armature. The presence of friction forces is particularly evident in gas-insulated circuit breakers, due to the kind of the electrical contacts in this type of circuit breakers.
Moreover, said friction forces are not generally symmetrical, and this entails the need to have different characteristics of the actuator when passing from the open position to the closed position and vice versa, while the holding force is relatively less important.
For these reasons, conventional actuators are oversized; this negatively affects the costs of the actuator, making it often more economical to resort to conventional spring-loaded mechanical actuators.